Every storage tank and transport tank has an access port. The access port may be large enough for a person to enter for the purpose of inspecting the interior of the tank or the access port may be only large enough to permit the passage of matter into the tank. With some commodities it is important that the tanks have port closures that can withstand a pressure differential between internal and external pressure. For example, in tanks used for the storage or transportation of granular or powder form dry bulk goods (such as flour, salt, cement, lime, and cereal grains) the contents are pneumatically transferred. Pneumatic transfer systems do not work properly unless the closure on the access port can maintain a pressure differential between interior pressure and exterior pressure.
The port closures in common use for such applications at the present time consist of an external lid. This external lid is held in position by a series of camlocks spaced evenly about a circumference of the access port. These closures have a number of problems. A first problem is that they are prone to leakage. A second problem is that the camlocks must be manually released. This requires a person to climb up onto the tank, and creates a potential safety hazard. The severity of the hazard depends upon the height of the tank, lighting conditions, weather conditions (wind, rain, etc), and other surrounding circumstances. A third problem is that care must be taken to ensure that the pressure within the interior of the tank is released prior to attempting to open the closure. If a pressure differential exists, the closure will open under pressure with a violent force. This violent opening force has resulted in a number of injuries and deaths as the person opening the closure has been struck by the closure and, in some cases, knocked off of the tank.